This invention relates to a method for obtaining an image signal by reading out a radiation image of an object from a recording medium, on which the radiation image has been recorded, and judging based on the image signal whether the irradiation field was limited during the recording of the radiation image.
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal heretofore have been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value designed so as to match the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a photograph or the like. In this manner, a visible image having good image quality with high contrast, high resolution high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays, or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and then is scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is detected photoelectrically converted into an electric image signal, and by using the image signal the radiation image of the object is reproduced as a visible image on a recording material such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor varies over a wide range and is proportional to the amount of energy stored during exposure to the radiation, it is possible to obtain an image having a desirable density, regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed, by setting an appropriate read-out gain when detecting the emitted light and converting it into an electric signal to be used in reproduction of a visible image on a recording material or a display device.
In order to detect an image signal accurately, certain factors which affect the image signal must be set in accordance with the dose of radiation delivered to the stimulable phosphor sheet and the like. A novel radiation image recording and reproducing system which accurately detects an image signal has been proposed in, for example, Japanese Unexamined Patent Publication Nos. 58(1983)-67240, 58(1983)-67241 and 58(1983)-67242. The proposed radiation image recording and reproducing system is constituted such that a preliminary read-out operation (hereinafter simply referred to as "preliminary read out") is carried out for approximately ascertaining the radiation image stored on the stimulable phosphor sheet. In the preliminary read out, the stimulable phosphor sheet is scanned with a light beam having a comparatively low energy level, and a preliminary read-out image signal obtained during the preliminary read out is analyzed. Thereafter, a final read-out operation (hereinafter simply referred to as "final read out") is carried out for obtaining the image signal, which is to be used during the reproduction of a visible image. In the final read out, the stimulable phosphor sheet is scanned with a light beam having an energy level higher than the energy level of the light beam used in the preliminary read out, and the radiation image is read out with the factors affecting the image signal adjusted to appropriate values on the basis of the results of an analysis of the preliminary read-out image signal.
The term "read-out condition" as used hereinafter means a group of various factors, which are adjustable and which affect the relationship between the amount of light emitted by the stimulable phosphor sheet during image read out and the output of a read-out means. For example, the term "read-out condition" may refer to a read-out gain and a scale factor which define the relationship between the input to the read-out means and the output therefrom, or the power of the stimulating rays used when the radiation image is read out.
The term "energy level of a light beam" as used herein means the level of energy of the light beam to which the stimulable phosphor sheet is exposed per unit area. In cases where the energy of the light emitted by the stimulable phosphor sheet depends on the wavelength of the irradiated light beam, i.e. the sensitivity of the stimulable phosphor sheet to the irradiated light beam depends upon the wavelength of the irradiated light beam, the term "energy level of a light beam" means the weighted energy level which is calculated by weighting the energy level of the light beam, to which the stimulable phosphor sheet is exposed per unit area, with the sensitivity of the stimulable phosphor sheet to the wavelength. In order to change the energy level of a light beam, light beams of different wavelengths may be used, the intensity of the light beam produced by a laser beam source or the like may be changed, or the intensity of the light beam may be changed by moving an ND filter or the like into and out of the optical path of the light beam. Alternatively, the diameter of the light beam may be changed in order to alter the scanning density, or the speed at which the stimulable phosphor sheet is scanned with the light beam may be changed.
Regardless of whether the preliminary read out is or is not carried out, it has also been proposed to analyze the image signal (including the preliminary readout image signal) obtained and to adjust an image processing condition, which is to be used when the image signal is processed, on the basis of the results of an analysis of the image signal. The proposed method is applicable to cases where an image signal is obtained from a radiation image recorded on a recording medium such as conventional X-ray film, as well as to the systems using stimulable phosphor sheets.
Various methods have been proposed for calculating how the read-out condition for final read out and/or the image processing condition should be adjusted on the basis of an analysis of the image signal (including the preliminary read-out image signal). As one of such methods, it has been proposed in, for example, Japanese Patent Application No. 59(1984)-12658 to create a histogram of the image signal. When a histogram of the image signal is created, the characteristics of a radiation image recorded on a recording medium such as a stimulable phosphor sheet or X-ray film can be ascertained based on, for example, the maximum value of the image signal, the minimum value of the image signal, or the value of the image signal at which the histogram is maximum, i.e. the value which occurs most frequently. Therefore, if the read-out condition for the final read out, such as the read-out gain or the scale factor, and/or the image processing condition such as the gradation processing condition or the frequency response processing condition is based on an analysis of the histogram of the image signal, it becomes possible to reproduce a visible image suitable for viewing, particularly for diagnostic purposes.
On the other hand, in the course of radiation image recording, it is often desirable for portions of the object not related to a diagnosis or the like to be prevented from being exposed to radiation. Further, when the object portions not related to a diagnosis or the like are exposed to radiation, the radiation is scattered by such portions to the portion that is related to a diagnosis or the like, and the image quality is affected adversely by the scattered radiation. Therefore, when a radiation image is recorded on the recording medium, an irradiation field stop often is used for limiting the irradiation field to an area smaller than the overall recording region of the recording medium so that radiation is irradiated only to that portion of the object which is to be viewed.
However, in cases where the read-out condition for the final read out and/or the image processing condition is calculated on the basis of the results of an analysis of the image signal in the manner described above and the image signal is detected from a recording medium, on which a radiation image has been recorded by limitation of the irradiation field, the radiation image cannot be ascertained accurately if the image signal is analyzed without the shape and location of the irradiation field being taken into consideration. As a result, an incorrect read-out condition and/or an incorrect image processing condition is set so that a visible radiation image suitable for viewing, particularly for diagnostic purposes, cannot be reproduced.
In order to eliminate the aforesaid problem, the applicant has proposed various methods for recognizing an irradiation field as disclosed in, for example, Japanese Unexamined Patent Publication No. 61(1986)-39039. The proposed methods allow the aforesaid problem to be eliminated by recognizing where the irradiation field lies on the recording medium, and calculating the read-out condition for the final read out and/or the image processing condition on the basis of only an image signal corresponding to the region thus recognized.
In general, in the disclosed methods for recognizing an irradiation field, several points which are considered to be present on a contour of the irradiation field, i.e. several prospective contour points, are detected. Thereafter, the straight lines or curves connecting the prospective contour points are detected, and the region surrounded by the straight lines or curves is recognized as the irradiation field.
A novel method for detecting a prospective contour point has been proposed in, for example, Japanese Unexamined Patent Publication No. 62(1987)-15538. The proposed method comprises the steps of detecting light emitted by the recording medium and thus obtaining an image signal composed of image signal components representing the image information at picture elements on the recording medium, and carrying out differentiation processing of the image signal components representing image information recorded at picture elements located along a single line on the recording medium. A point at which the absolute value of the differentiated value obtained during differentiation processing exceeds a predetermined threshold value is detected as a prospective contour point. In cases where several such points are present, the point nearest to an edge of the recording medium is detected as a prospective contour point.
In cases where the irradiation field first is detected and then the image signal representing the image information recorded in the region inside of the detected irradiation field is analyzed in the manner as that described above, an appropriate read-out condition and/or an appropriate image processing condition is determined.
However, with the conventional techniques, because a substantially long operation time is required to detect the irradiation field, the processing capacity of the system is low. Though various attempts heretofore have been made to shorten the operation time, the operation time cannot be shortened too much in order to detect accurately the irradiation field.
On the other hand, many radiation images are recorded without the irradiation field being limited. However, whether the irradiation field was or was not limited heretofore has been known only after the operation which detects the irradiation field is carried out. Therefore, the operations which detect the irradiation fields heretofore have been carried out for all radiation images regardless of whether the irradiation fields were or were not limited during the radiation image recording.